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Fabrication of Electroplated Stainless Steel Based Eutectic Sn/Pb Solder Decals

IP.com Disclosure Number: IPCOM000114344D
Original Publication Date: 1994-Dec-01
Included in the Prior Art Database: 2005-Mar-28
Document File: 4 page(s) / 217K

Publishing Venue

IBM

Related People

Venkatraman, R: AUTHOR

Abstract

A decal is a device for transferring controlled amounts of solder onto an electronic carrier. The device was developed in [1,2] for chip attach using eutectic Sn-Pb solder for direct chip attach applications. The decal transfer process is schematically shown in Figs. 1 and 2. The substrate material for such a device is a non-wettable substrate such as stainless steel. To transfer onto the C4 copper pads of the carrier, the decal (which has eutectic solder plated with the same footprint as that of the carrier) is placed using a vision system onto the carrier and the assembly reflowed at a temperature above the eutectic melting temperature of the solder (183 degrees C). The solder dewets off the substrate surface and attaches to the copper pads of the carrier.

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Fabrication of Electroplated Stainless Steel Based Eutectic Sn/Pb
Solder Decals

      A decal is a device for transferring controlled amounts of
solder onto an electronic carrier.  The device was developed in [1,2]
for chip attach using eutectic Sn-Pb solder for direct chip attach
applications.  The decal transfer process is schematically shown in
Figs. 1 and 2.  The substrate material for such a device is a
non-wettable substrate such as stainless steel.  To transfer onto the
C4 copper pads of the carrier, the decal (which has eutectic solder
plated with the same footprint as that of the carrier) is placed
using a vision system onto the carrier and the assembly reflowed at a
temperature above the eutectic melting temperature of the solder
(183 degrees C).  The solder dewets off the substrate surface and
attaches to the copper pads of the carrier.

      Decal Requirements - It is apparent that in order to obtain
good transfer yields, the following conditions must be met.
  1.  The substrate material must completely dewet the molten solder
       otherwise substantial forces will be required to separate the
       decal from the carrier after reflow.  This will result in
       separation within the solder or sometimes, in the tearing of
       copper lands from the carriers if excessive forces have to be
       used.  Stainless steel, chromium and titanium are examples of
       metals that do not wet molten solder [3].
  2.  Secondly, the solder bumps must adhere to the substrate in the
       as-plated form.  Thus, the substrate must offer sufficient
       mechanical adhesion so that the plated bumps are not lost
during
       subsequent mechanical handling and processing.
  3.  Thirdly, the copper surface must be clean enough to wet the
       molten solder.  Commonly, the solderability of the copper is
       preserved by passivating the surface with benzotriazole
(commonly
       known by the brand name "Entek").

    The other requirements for a decal are:
  4.  Consistent solder volume.  This is largely a function of the
       consistency in photoprocessing and in the ability to plate
       eutectic tin-lead without solder voids.
  5.  Solder composition:  The composition of the plated solder bumps
       must be close enough to the eutectic composition (for example
       63+/- 5 wt.%Sn) that the bumps are completely converted to
liquid
       at the reflow temperature.  As the temperature deviates from
the
       eutectic, higher temperatures are required to obtain 100%
liquid
       as dictated by the Sn-Pb phase diagram.  Additionally, the
solder
       must not contain any impurity elements that will increase the
       wettability to the substrate and thus leading to non-release.
  6.  The solder bumps must be free of contaminants such as residual
       resist...